In our copending patent applications Ser. Nos. 685,246 and 788,611, identified above, coating compositions are described that comprise a compatible blend of filmforming polymeric constituents in a liquid vehicle wherein the polymeric blend is, in a preferred embodiment of the invention, an associatively-formed mixture of a carboxylic acid-functional graft polymer and carboxylic acid-functional addition copolymer. In the embodiments of the invention of our copending applications, Ser. Nos. 685,246 and 788,611, the graft polymer has an epoxy resin component that has an integral aliphatic carbon chain, onto which is grafted at aliphatic carbons thereof an addition copolymer component that contains carboxylic acid units derived from a carboxylic acid monomer, which units constitute at least 1% by weight of the graft polymer. The grafting amounts to at least 11/2 parts by weight of the addition copolymer component for each 100 parts by weight of the epoxy resin component. The acid number of the blend falls in the range from about 30 to about 220.
Such copending applications Ser. No. 685,246 and Ser. No. 788,611, also describe processes for making such coating compositions. One such process involves reacting an epoxy resin with a mixture of monomers containing ethylenic unsaturation, which monomers are copolymerizable to form the addition copolymer. These materials are reacted in the presence of at least 3% of benzoyl peroxide by weight based on the weight of the mixture of monomers, at about 110.degree. C. to about 120.degree. C. Instead of benzoyl peroxide, any other free radical initiator may be used that furnishes equivalent free radical initiating activity for this reaction at that temperature. The epoxy resin reactant generally amounts to from 5% to 95% of the initial reaction mixture, preferably at least 50%, and for highly demanding can coating compositions, from 60% to 90%.
In those processes of Ser. No. 685,246 and Ser. No. 788,611, the presence of the unusually large amount of free radical initiator, based on the weight of polymerizable monomer, together with the strong orienting influence of the epoxy resin groups, favored the formation of a graft polymer in which the grafting was predominantly on aliphatic backbone carbon atoms of the epoxy resin. Some grafting, in minor amount, occurred at other locations as well. Grafting of other types occurred, particularly ester grafting in which carboxylic acid groups reacted with epoxy groups. Ester-type grafting is less desirable than aliphatic carbon-to-carbon grafting because the linkage is less resistant to hydrolysis and weathering.
The copending application Ser. No. 788,454 spoke of capping the epoxy resin, the term "capping" being used in a broad sense wherein some or all of the oxirane (epoxy) groups of the starting epoxy resin are eliminated. Such capping is not reversible in the sense that the word "capping" sometimes is used in connection with some other organic reactions, and the resulting "caps" cannot be removed readily to restore the original oxirane configuration. Hence the word "terminating" is believed to be better here than "capping" to describe this oxirane elimination.
The "capping" or termination can be done to oxirane (epoxy) groups on the starting epoxy resin either without extension of the starting epoxy resin molecules (eg. using benzoic acid), or with extension which might even be substantial (eg., using oxirane-terminated epoxy resin reacted with more or less of a diphenol such as bisphenol A). In such non-extension instance there are, of course, residues of the carboxylic acid projecting from where eliminated oxirane groups were; in such extension instance, the resin chain becomes generally much longer as more such diphenol reactant is used, and less long as less such diphenol is used.